1. Field of the Invention
The present invention generally relates to image forming apparatuses and methods, and more specifically to an image forming apparatus and method with improved capabilities of correcting image magnification error without prolonging non-writing periods and unduly affecting print speeds.
2. Discussion of the Background
In increasingly versatile image forming apparatuses, various improvements have been made in the area of image qualities to cope with severe requirements.
In an image forming apparatus such as a copying machine, a copier, a facsimile, and a printer inclusive of their color models, light beam scanning systems are utilized for optical data writing.
The electrophotographic image forming process utilizing such a light beam scanning system is well known, in which the formation of the images is performed through the electrophotographic process with a light beam by modulating according to image data, deflecting with a constant angular velocity in the horizontal (primary) scanning direction by a rotating deflecting unit (polygon mirror), converting while properly adjusting to constant linear velocity by an f-θ lens, and scanning the light beam over the surface of an image bearing member (photoreceptor).
With regard to the light beam scanning systems, however, several drawbacks have previously been encountered such as, for example, undue variation of image magnification between the systems, which is caused by the variation or dispersion of characteristics of the system (e.g., lens). The dispersion may become evident when a plastic lens is employed in the beam scanning system. This arises from relatively large changes in shape or refractive index primarily with the change in its ambient temperature.
This gives rise to undesirable positional shifts of beam scanning on the surface of the photoreceptor, that is, deviations of image magnification (which is hereinafter also referred to as errors) in the horizontal scanning direction. As a result, several difficulties are encountered in achieving high image qualities.
Furthermore, in the case of forming multiple-color images by the image forming apparatus incorporating multiple lenses for transmitting plural laser beams, the dispersion may become more pronounced by accumulating a shear in color printing (out-of-register color) caused by the errors of image magnification from respective lenses, whereby image qualities are deteriorated especially in color, in this case.
Therefore, it would be desirable to properly adjust the image magnification for respective colors to the extent possible.
Several disclosures have previously been made to obviate the difficulties that arise from the above-mentioned changes in system characteristics with the change in its ambient temperature and other factors, which may be achieved by adjusting image magnification in the horizontal scanning direction.
For example, in Japanese Laid-Open Patent Application No. 2001-180043, a method is disclosed in which the accuracy of magnification correction may improve by detecting a light beam deflected by a polygon mirror at least at two separate locations on the line in the horizontal scanning direction, measuring the difference in time of passage between these locations, and suitably changing the frequency of a pixel clock and the speed of rotation of a polygon mirror, whereby corrections of image magnification are made without further subdividing the incremental step of a writing clock.
Another method is described in Japanese Laid-Open Patent Application No. 2000-229444 also for correcting image magnification in the horizontal scanning direction with an improved accuracy.
In this method, when the frequency of a pixel clock is controlled by a minimum incremental step to correct the image magnification, each dividing ratio of two frequency dividers included in a pixel clock generator is properly adjusted during the control of the frequency.
As a result, the correction is carried out with a high accuracy without deteriorating the jitter of a video clock and the image magnification corrections in the horizontal scanning direction can be performed by a relatively inexpensive structure with a high degree of accuracy.
In addition, still another method is disclosed in Japanese Laid-Open Patent Application No. 10-213940, in which test patterns are formed for detecting positional deviation between pages and the amount of the deviation is obtained by a detector. Based on the thus obtained amount, corrections between pages can be made several times to keep the color shear in printing to a minimum without impairing the efficiency in image processing.
FIG. 17 is a timing diagram illustrating the timing of output signals for making magnification corrections between pages (recording sheets) in a previous image forming apparatus configured to form a full-color image by superposing four monochromatic images.
Referring to FIG. 17, a signal XSTART is output by pages for starting a print operation, and XFGATE signals are subsequently generated, based on the signal XSTART, for determining the timing of writing images in respective colors, Y, M, C, and K. The print operations are each initiated at the point of time when the signal XSTART turns low (L).
In the above-mentioned method disclosed in the application No. 2001-180043, the speed of rotation of polygon mirror is adapted to change for making image magnification corrections.
When an image forming apparatus, which is configured to form four-color images with one polygon mirror, makes magnification corrections between pages in the manner illustrated in FIG. 17, it should be noted that the steps of the correction can be initiated only after completing a printing operation.
Also, in the case when multiple polygon mirrors are provided for use in respective colors, the next writing operation has to begin after changing, and subsequently stabilizing the number of rotation for the mirrors. In those cases, therefore, the interval between the pages has to increase to properly perform magnification corrections.
In addition, in the case when both the detection and correction of the magnification error are made within a single page, it takes more time and the interval between the pages increases accordingly, since the data obtained from the detection have to be reflected to, and utilized for the correction.
In the method described in the application No. 2000-229444, image magnification corrections are made by generating pixel clocks with a PLL circuit and changing its dividing ratio.
Since the circuit generally takes time from (a) setting a dividing ratio to (b) reaching a predetermined value of the pixel clock frequency, it is difficult to properly change the frequency within the time period between pages during a continuous printing operation, and the magnification corrections are carried out by increasing the interval between the pages, as illustrated in FIG. 17.
When both the detection and correction of the magnification error are intended within a single page, it takes more time and the interval between the pages increases accordingly.
Since this increase in the interval may unduly affect the printing speed, the interval of the magnification correction has to increase, that is, the frequency of the correction decreases. Therefore, when the corrections are carried out as above, the deviation in image magnification becomes significant as illustrated in FIG. 18, whereby out-of-register colors appear noticeable in resulting images.
Furthermore, although test patterns are formed for detecting positional deviation between pages in the method disclosed in the application No. 10-213940, this gives rise to another drawback, such as an increased amount of toner consumption in every pattern formation.
In addition, since the corrections are made step by step with the pages, it takes time to complete the correction. This prolonged time may not be so problematic as long as any appreciable deviation can be prevented. It is desirable, however, for the former difficulty in toner consumption to be obviated as much as possible, because the toner consumed by pattern printing may be considered as other than necessary by a user.